Fix conditional dependency on futures

setup.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 
+import sys
 import ast
 import re
 
@@ -11,6 +12,13 @@ with open('click_threading/__init__.py', 'rb') as f:
     version = str(ast.literal_eval(_version_re.search(
         f.read().decode('utf-8')).group(1)))
 
+INSTALL_REQUIRES = [
+    'click>=5.0',
+]
+
+if sys.version_info[0] == 2:
+    INSTALL_REQUIRES.append('futures')
+
 setup(
     name='click-threading',
     version=version,
@@ -20,10 +28,5 @@ setup(
     url='https://github.com/click-contrib/click-threading',
     license='MIT',
     packages=['click_threading'],
-    install_requires=[
-        'click>=5.0',
-    ],
-    extras_require={
-        ':python_version < "3.2"': 'futures'
-    }
+    install_requires=INSTALL_REQUIRES
 )